1. Field of the Invention
This invention relates to semiconductive devices and more particularly, to a method for fabricating a semiconductive device which is useful in application to high speed and high density integrated circuits.
2. Description of the Prior Art
In the MIS (metal-insulator semiconductor) type device using titanium silicide as part of the gate structure, e.g. a polycide gate structure, it is usual to subject the device to thermal treatment in an atmosphere of nitrogen after formation of the silicide in order to make wirings of low resistance. The main purpose for this is to lessen resistance caused by the growth of crystals of titanium silicide, to activate the source and drain regions of transistor, and to make a flat interlayer insulating layer by re-flow of the layer.
In the prior art technique, however, when the thermal treatment is effected in an atmosphere of nitrogen at temperatures not lower than 900.degree. C., titanium silicide which is thermally instable and thermally shrinkable suffers morphological degradation, resulting in an increasing resistance of the wirings. This morphological degradation is described, for example, in Applied Physics Letter 48 (23) p. 1591 (1986). In addition, there is produced a great stress accompanied by the morphological degradation, causing a large quantity of interface levels to be produced in the interfaces of the MIS semiconductor and thus degrading the transistor characteristics.
For preventing the morphological degradation during the thermal treatment of titanium silicide, there are known two improvements: (1) an impurity is incorporated in the titanium silicide such as by ion implantation thereby improving the thermal stability of the silicide; and (2) an insulating film is formed on the titanium silicide film.
As for (1), nitrogen implantation into the silicide is described in Appl. Phys. Lett. referred to above, by which the stability of the silicide is stated as improved.
The formation of an insulating film on the titanium silicide film is described with reference to FIGS. 1a to 1d.
In FIG. 1a, there is shown a microphotograph showing morphological degradation of a titanium silicide film surface which has been subjected to thermal treatment at 900.degree. C. for 120 minutes in an atmosphere of nitrogen without covering with an NSG (non-doped silicide glass) film prior to patterning to form a polycide wiring. FIG. 1b is a microphotograph showing morphological degradation of a titanium silicide film surface which has been subjected to thermal treatment in an atmosphere of nitrogen under the same conditions as stated above but covered with a 150 nm thick NSG film on the film surface. In FIGS. 1c and 1d, there are, respectively, shown sectional views taken along the lines a.sub.1 -a.sub.2 and b.sub.1 -b.sub.2 of FIGS. 1a and 1b. As will be apparent from FIG. 1c, the thermal treatment in a nitrogen atmosphere makes an irregular titanium silicide surface although the covering with the NSG film mitigates the degree of irregularily to an extent as is particularly shown in FIG. 1d.
However, the surface morphology is not improved by these known techniques to a satisfactory extent.